CLASSIFICATION OF STBUCTURAL STEELS.
per cent, of phosphorus may be allowed; but in railroad bridges, traveling cranes, and other structures where the steel is exposed to moving loads and continued shock, and where the consequence of failure may not be measured in money, the specifications should require the use of open-hearth steel. The phosphorus limit in acid steel should be .08 per cent, and in long span bridges it should be .06 per cent. In basic steel it should always be below ,04 per cent.
It is necessary also to specify the manner in which the sample shall be taken lor analysis. There are four methods of doing this, qf which only one is correct, and this correct one is seldom or never used. Taking for illustration a rolled billet of steel three inches square, its cross-section may be mentally divided into nine equal, squares, each having an area of one square inch. Eight of these sqiiares are next to the surface, while one is in the interior. This central sqiiare will include the segregated portion of the mass; Ordinarily a sample of such a billet would be taken by drilling to a depth of half an inch, but this does not touch the interior core, and, the chemical determinations will show too low a content of segregating metalloids.
Another method is to drill to the center, and take all the drillings that are made. Two-thirds of these drillings will come from the outside squares and one-third from the inside, or a ratio of two from the outside and one from the interior, while the true ratio is eight to one; hence the content of segregating metalloids found by this method is higher than the true average.
A third method is to take, drillings from the central portion, but this will, give a higher content of certain elements than will be found throughout the bar. •
The fourth way is to plane the entire surface and get a true average, but this practice is seldom carried out. ,
In the case of angles, a fair sample can be obtained by drilling into the bar as far as, the center;, the-results being only slightly higher than the true values. In plates it is more difficult to take # fair sample, since the segregated portion is in the body ,of the sheet, and1 it is usually impracticable to drill a hole without injure ing the member. Great injustice, may be; .done- by unusual methods of sampling. .It would be perfectly light to state in the contract that drillings^ were to be taken from, the center of 1ihe< plate, hut it .is not righfe to take'them in" this; way .in. the absenceof •:&••,